1. Field of the Invention
This invention relates to flexible laminates and to a process for their production and more specifically to tissue laminates reinforced with a web having fibers treated with a finish so that the tissue laminates have excellent levels of tear strength.
2. Description of the Prior Art
Flexible laminates of fiber layers are well known in the art. These products generally comprise reinforcing webs and layers of various fibers. The reinforcing web can be used as a centrally disposed layer with adhering outer fiber layers or a pair of reinforcing webs can be used to sandwich a centrally disposed fiber layer. A typical laminate of the prior art is described in U.S. Pat. No. 3,388,028 issued to Robert R. Alexander on June 11, 1968. The laminate described in this patent comprises a layer of cellulose fibers plied between a pair of reinforcing webs.
A specific class of flexible laminates are known as tissue laminates. These products comprise a central reinforcing web and outer tissue layers. Tissue laminates are drapable, strong and absorbent and have gained widespread use as disposable clothing, toweling, and surgical drapes.
The laminates of the prior art have generally been prepared by impregnating a central reinforcing web having an open mesh with an adhesive and then placing tissue layers on the upper and lower surfaces of the web. The tissue layers become bonded to the reinforcing web and to one another through the openings in the web. These tissue laminates are intended to be used as inexpensive substitutes for conventionally woven cloth fabrics. It is a goal of those skilled in the art to achieve cloth-like properties which are characteristic of woven fabrics such as drape, strength and absorbency while at the same time supplying a cost saving substitute.
Laminate strength is generally attributed to the central reinforcing web. This web can be knitted, woven or nonwoven. If a nonwoven web is used it may be formed by spunbonding, fiber entanglement or needle punching. The fibers used in reinforcing webs may be composed of a natural or synthetic substance such as cotton, wool, polyamide, polyester or blends of such substances. The particular fiber selected for reinforcement and the construction of the web are significant factors in the over-all strength properties of the laminate. In addition to strength a laminate is also characterized by aesthetics, i.e., drape, softness and absorbency. These properties are generally attributed to the characteristics of the tissue layers, and the type and quantity of adhesive used for bonding the layers in the laminate.
Tissue laminates having adequate strength and offering cloth-like properties have been produced with varying degrees of success. Some laminates have been successfully substituted for conventional woven fabrics. Prior workers have modified laminate properties by varying the type and quantity of adhesive, the construction of the reinforcing web and/or the type of tissue layer. To achieve the best combination of properties, i.e., strength without a sacrifice in laminate aesthetics namely drape and softness, adhesive concentration must be carefully controlled. A soft and drapable product can be attained with lesser amounts of adhesive but delamination of the individual layers will generally occur. On the other hand, a higher concentration of adhesive will prevent delamination but there will be a sacrifice in laminate aesthetics.
U.S. Pat. No. 3,629,047, issued to Robert W. Davison on Dec. 21, 1971, recognized that the strength of laminates and particularly tear strength can be enhanced by preventing or minimizing the adherence of the central reinforcing web to the outer layers. This patent discloses that when an external force of sufficient strength is applied to a laminate a tear will occur. This tear will continue unabated throughout the laminate if the individual fibers in the reinforcing web are restrained and prevented from moving about in order to relocate and redistribute the applied stress. Failure of the laminate occurs because the individual fibers fail one at a time. Therefore the only force required to initiate a tear is that needed to break a single fiber. The patentee goes on to disclose that if the individual fibers in the reinforcing web have freedom of movement they can bunch up and reinforce one another at the tear point and thus offer resistance to the continued propagation of the tear. Thus, laminates should be assembled in such a manner that the fibers in the reinforcing web have freedom of movement. This patent discloses that the fibers in the web should be unbonded or only lightly bonded to one another. Secondly when the laminate is formed the fibers in the web should not adhere to the outer layers. The patentee states this can be accomplished by using an adhesive that will adhere to the fibers of the outer layers but not to the fibers of the reinforcing web.
U.S. Pat. No. 3,862,877, issued to James B. Camden on Jan. 28, 1975, discloses a cloth-like tissue laminate having improved drape. This patent discloses a variety of materials suitable for a reinforcing web. The patentee desires that the fibers of the reinforcing web be free to move against one another at their crossover points. The patentee states this can be accomplished when, "the tissue face layer on the reinforcement side of the laminate is combined with the central impregnated tissue layer in a pocket bonding fashion to surround, but not bond, the strands of the scrim reinforcement to the facing tissue layer." (Col. 1, lines 26-30). By the term "pocket bonding" the patentee is believed to mean the web fibers are enveloped, but not bonded to, the outer tissue layers. It is the patentees belief that the reinforcing fibers are thus assured freedom of movement by virtue of pocket bonding and a laminate having improved properties is thereby obtained.
In general workers in the field have recognized that laminates, especially tissue laminates, having adequate strength and capable of resisting tear would be commercially attractive as alternative materials for conventionally woven and/or knitted fabrics. It has been found that laminates having the property of resistance to tear can be obtained by using a reinforcing web having strong individual fibers capable of substantially unrestricted movement when the laminate is stressed.
Although Davison and Camden recognized that the fibers constituting the reinforcing web must have substantial freedom of movement the accomplishment of such a result has proven to be elusive in the actual fabrication of flexible laminates and particularly tissue laminates. Many well known adhesives such as water-insoluble polymer dispersions have caused adhesion to take place between the fibers of reinforcing webs and the outer tissue layers. This has resulted in restricting fiber movement within the laminate with a resultant decrease in laminate strength properties, particularly tear strength.